Hi,
how do you change the way emacs selects regions of text? Currently it works
line based, but I need to select a rectangular region from within a text.
Can anyone help me with how to change the selection mode?

If you use cua mode it has a rectange editing mode.
In cvs emacs this is in cua-rect.el.
I find it easier to use than the "standard" rectange functions.
Paul